Traffic congestion often begins at restrictive points, such as bridges, tunnels, and construction sites, where multiple lanes of traffic merge into a single lane, or where drivers must maintain travel in a lane through a narrow area (causing many drivers to slow down). While a little congestion may be harmless, around heavy-traffic times this can snowball into significant congestion, suddenly taking hours to sort itself out.
It has been observed that using different vehicles per minute (flow) passing a point, with varied given number of vehicles per mile, optimal flow can be achieved at somewhat less than a maximum number of vehicles per mile (bumper to bumper traffic). That is, if there were a way to control a number of vehicles spaced out over a given interval (such as a mile) and limit that to an optimal or near optimal number, and also to control the vehicle speed, traffic could continue to flow without hitting a point where no one can go anywhere (stopped, bumper to bumper traffic). Of course, it is nearly impossible to control the speed at which each driver drives, and to prevent vehicles from stacking up close to proceeding vehicles, such that traffic, in terms of vehicles per mile, over-reaches the optimization point.
One attempt at controlling flow includes installing lights or other traffic control devices at certain points. This requires substantial infrastructure, slows traffic unnecessarily at certain points to optimize flow at other points (by, for example, stopping each vehicle for a second or two), and can result in public annoyance against devices which may actually provide some advantage, but are perceived by the public as onerous and unnecessary.